My Lawnmower Has No Compression at All

What Is Compression?

• When fuel flows into the combustion chamber, the spark plug discharges a high-voltage spark, igniting the fuel. This explosion generates enough force to push the piston down inside the cylinder. As the piston moves down and up, it also turns the crankshaft sitting underneath it. When the crankshaft spins fast enough, it generates a vacuum, which allows the fuel pump to bring fuel into the carburetor. This increase in pressure keeps the combustion process going.

Check the Recoil Starter

• The recoil starter on a lawnmower allows the operator to manually generate enough compression for the combustion process to get started; once started, the process continues on its own. A broken recoil starter system may appear to exhibit similar symptoms as a total loss in compression. Before assuming the lawnmower has lost compression, thoroughly inspect the starter rope, the starter rotor, the starter's recoil spring and the flywheel's fins, and replace any damaged parts.

Test for Compression

• A simple test of the engine's vacuum pressure can tell you whether the mower still has compression or not. A compression gauge, which looks much like a tire gauge, connects to the engine's cylinder. After you pull firmly on the rope until the needle stops moving, this gauge will give you a reading of the amount of compression available in the engine. Most lawnmowers will need at least 60 psi to operate. However, a good mower engine will run with more than 100 psi.

Compression Loss

• Losses in compression usually occur from air leaking into the internal engine, where compression is most needed. The crankshaft extends out of the crankcase and attaches to the blade and any other moving parts. To keep this area sealed from outside air, a gasket is necessary. However, if the gasket starts to crack or break, air will get in, and the engine loses compression. When air leaks into the system, most of the engine will need breaking down to check all of the sealing points, which can be expensive and time-consuming.